Re: [Xen-devel] rdtscP and xen (and maybe the app-tsc answer I've been looking for)

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 21/09/2009 17:55, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> Both Intel and AMD have confirmed that constant_tsc means
> that TSC is consistent across all cores and even across
> multiple sockets; and at least one major system vendor (HP)
> with multi-enclosure "big iron" AMD-based NUMA systems has
> confirmed that TSC is consistent across all nodes.   So
> by applying the Xen rendezvous-sync algorithm (that writes
> tsc every second) on such machines, Xen has actually been
> creating a tsc-sync problem, not alleviating one!

Constant_tsc is not even directly a hardware flag. It's a synthetic value
that Linux derives for itself and we inherited. Are vendors really making
guarantees about a flag which they do not directly provide?

 -- Keir




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